Chemical Speciation and Bioavailability of Selenium in Selected Saskatchewan Soils
Exner, Sheldon Wayne
Selective sequential extraction techniques that partition soil selenium (Se) into different particulate-bound fractions have been developed to provide information regarding the geochemical and soil processes controlling Se transformation, dynamics and bioavailability within the soil environment. However, none of these earlier extraction techniques considered the fraction of Se bound to metal-organic complexes, a very physicochemically reactive soil component. The objectives of this study were to: (1) determine the distribution of particulate-bound Se fractions (including Se bound to metal-organic complexes) within selected soil profiles developed on undifferentiated glacial till parent material in the major soil zones of Saskatchewan and currently used in agricultural production; and (2) relate the occurrence and distribution of the particulatebound Se fractions to a Se bioavailability index (SBI). Concentrations of total Se and the SBI in the soil profiles studied ranged from 0.263 to 1.295 mg kg-1 soil and 0.006 to 0.047 mg kg-1 soil, respectively. Total soil Se was partitioned into seven particulate-bound fractions: water soluble, exchangeable, carbonate-bound, metal-organic complex-bound, amorphous mineral colloid-bound, organic-bound and residual. The percent recovery of total Se ranged from 86.8 to 112.2%, with an average value of 96.8%. The water soluble, exchangeable, carbonatebound, and amorphous mineral colloid-bound Se fractions accounted for 1.0 to 3.9%, 0.3 to 4.4%, 3.0 to 4.6%, and 0.5 to 4.0% of the total soil Se, respectively. The metalorganic complex-bound, organic-bound and residual Se fractions accounted for 7.4 to 37.5%,6.8 to 20.7%, and 44.2 to 77.4% of total soil Se, respectively. Thus, the results indicated that metal-organic complex-bound Se accounted for a significant proportion of total soil Se. Further, the results have provided the first information pertaining to the distribution of particulate-bound Se fractions in Canadian soils. Water soluble, metal-organic complex-bound and organic-bound Se fractions were significantly correlated with the SBI; thus, any of these three Se fractions could potentially contribute to bioavailable Se. Because water soluble Se accounted for a small proportion of total soil Se and was much less than the SBI in the vast majority of the soil horizons of the solums, the results suggest that metal-organic complex-bound Se could play a relatively significant role in contributing to bioavailable Se.